Parallel four-bar mechanism is a commonly used kinematic mechanism and is widely adopted in medical and diagnostic equipments, such as ultrasonic diagnostic instruments. A parallel four-bar mechanism comprises a first bar, a second bar, a third bar and a fourth bar. The first bar forms a fixed seat, the fourth bar forms a connecting seat to be mounted to a device or an apparatus, and the second and third bars are parallel to each other and are hinged to the first and fourth bars. The parallel four-bar mechanism forms a planar parallelogram, wherein the first to the fourth bars forming four sides of the parallelogram. The parallel four-bar mechanism has a degree-of-freedom for pitching movement and can be used as a supporting arm for a display of an equipment, so as to raising or lowering the display with the posture of the screen of the display remaining unchanged. The parallel four-bar mechanism thus used has a disadvantage. Specifically, no locking means is provided between the second and third bars, and thus translational movement may happen between the second and third bars. In some circumstances, such as for facilitating the handling and transportation of the display, it is difficult to keep the display in a predetermined position without being raised or lowered.